I managed to take a photograph of each one, though unfortunately from opposite sides. I've studied then for well over five minutes now and can't make up my mind, so over to you. For what it's worth I think the streaking on the head seems very similar towards the front, more or less similar around the eyes - though I am wondering, with the bird's posture similar in each shot - whether the dark streak behind the eye isn't at a different angle on each bird. Hey ho. Moving on, a lot less similar at the back, on the nape, where the January bird (facing right) seems far darker. That said, there is a faint yet obvious whitish divide/collar between the really dark streaking on the rear of the neck and the side of the face on both birds, though it appears more distinct on the November bird (facing left). Bill colour is an obvious difference, but I can't positively say that January bird hasn't just got a bit muddy.
I'm not going to go into Ps, as frankly I have no idea what they are, but if you look at the tip of P27, you'll see a little bit of white. Is that right? Almost impossible to compare as we're looking at a different side of each bird, and they're not necessarily a mirror image. I reckon they're pretty much the same, though I can't see that darker feather on the upper back of the November bird as being present on the January bird, or at least not on the side I can see.
The other problem is that the light was dreadful on January 1st - conditions so dire that the bird looks purple - just look at the BH Gull next to it, they look almost the same shade of purple. Making any comparisons of the mantle colour is fraught with danger. So, the two bird theory? I'm almost convinced, but not quite. We do have a lot of Common Gulls on the Flats, and I'm sure that the population has a fair bit of turnover as I don't, for instance, see the scabby white-winged one every time, nor this/these darker-headed bird/birds. Could there really be two birds with ridiculous streaking like this 40 days apart. Well, very likely there could. A horrible thought indeed. Much better is this one. Spot on I'd say.